The present invention relates generally to position measurement devices in general and more particularly to devices used to sense linear motion using magnetic fields.
It is often necessary to measure the position or displacement of two elements relative to each other. This displacement can be measured with many different sensing technologies over a large range of accuracies, with different levels of complexity, and at a wide range of costs.
Some common apparatus for measuring linear displacement employ linear encoders, capacitive sensors, eddy current sensors, a linear variable differential transformer, photoelectric or fiber optic sensors, or magnetic field sensors. Linear encoders use a glass or metal ruler that is made of a high stability material so that changes in temperature do not affect measurement accuracy. These materials, such as quartz, steel, Invar® alloy, glass or ceramics generally require special machining techniques to manufacture and thus are more expensive.
Capacitive sensors are used with both conductive and nonconductive target materials but are very sensitive to environmental variables that change the dielectric constant of the medium between the sensor and the target, usually air. Eddy current sensors contain two coils: an active coil that indicates the presence of a conducting target, and a secondary coil that completes a bridge circuit. A linear variable differential transformer (LVDT) sensor has a series of inductors in a hollow cylindrical shaft and a solid cylindrical core. The LVDT produces an electrical output that is proportional to the displacement of the core along the shaft. The size and mounting of these coils or cores and the sensitivity of measurement are competing design factors in the use of eddy current or LVDT sensors.
Photoelectric and fiber optic sensors use beams of light to measure distance or displacement. The photoelectric sensor uses free-space transmission of light while the fiber optic sensor uses a pair of adjacent fibers to carry light to a target and receive reflected light from the object. Alignment of the fibers and the complexity of the optics needed to maintain the light path are difficulties in using this technology.
Magnetic sensors such as the Hall effect sensor, GMR sensor, or an AMR sensor can be used with a linear array of teeth or alternating magnetic poles to produce a sinusoidal output indicative of the sensor's linear motion. However, the initial position must be determined and each tooth or magnetic pole must be counted and phase data analyzed for greatest accuracy.
A sensor which outputs voltage which is directly proportional to linear position has the advantage that it may be turned on and may produce an accurate determination of position without calibration or reset. One such sensor uses a pair of magnets with convex surfaces of the same magnetic pole facing each other. However, this type of sensor requires forming a nonlinear curve on the faces of the magnets which, depending on the magnetic material used, can be costly. My own earlier invention U.S. Pat. No. 7,521,922 uses stepped magnets to produce a substantially linear varying magnetic field. However sensors with longer linear fields, and resistant to a wide range of temperatures are desirable.
What is needed is a magnetic linear displacement sensor which produces direct correspondence between position and magnetic field strength that can be constructed with a simple magnet geometry and which can operate at higher temperatures.